Semiconductor devices are used in a wide variety of applications, for example, in cellular phones and personal computers. The high demand for portable devices as well as advances in semiconductor process has created new state of the art processes that must be operated at low power supply voltages.
As semiconductor feature sizes get smaller and smaller, the voltage level that these devices can withstand has decreased correspondingly. Thinner gate oxides and shorter channel lengths have reduced common supply voltages from the 5V and 3.3V seen a decade ago to 1.2V and below. The higher device density and faster performance of submicron processes have come at a cost of lower device breakdown voltages. High demand for small portable devices such as MP3 players have also increased the demand for circuits that can operate efficiently on a single battery cell.
Lower power supply voltages have posed a number of circuit design challenges and difficulties. One of these difficulties lies in on-chip reference voltage generation. Regardless of the particular application, most semiconductor circuits require accurate and predictable bias generation in order to guarantee acceptable circuit performance over a process, temperature and power supply voltages. Bandgap voltage references, in particular, are widely used to generate temperature independent voltages. These temperature independent voltages are then used, for example, to derive A/D thresholds, regulated power supply voltages, and temperature independent current sources. Bandgap voltages are derived by summing a diode voltage (typically between about 0.6V and 0.8V) which has an inversely proportional to temperature characteristic, to a voltage that is proportional to temperature. The resulting voltage is typically about 1.23 volts in a silicon process in order for the positive and negative temperature characteristics to cancel out. Because the resulting voltage is in the neighborhood of the 1.11 bandgap of silicon, these references are commonly referred to as bandgap references.
As supply voltages decrease to 1.6 V and below, circuits commonly used in the past to generate reliably a 1.23V temperature independent voltage are no longer feasible. Some circuits have been developed to address this issue by using voltage scaling and sampling techniques, however, many of these circuits suffer from noise, inaccuracy, low yield, and switching ripple. In the field of semiconductor circuits, what are needed are high-yielding, low voltage bandgap circuits that provide accurate, low-noise and low-ripple outputs.